1. Field of Invention
This invention relates generally to hatch covers, and more particularly to a gasketed hatch cover for use in an odor control system or other installation having a hatch opening which when closed by the gasketed cover is then sealed to prevent the escape of fumes.
2. Status of Prior Art
A gasketed hatch cover in accordance with the invention is applicable to any installation in which there is a hatch opening from which fumes emanate, the hatch cover when closed serving to seal the opening to prevent the escape of the fumes into the atmosphere. While the invention will be specifically described in connection with a wastewater treatment plant provided with odor control means, it is to be understood that the invention is by no means limited to this particular application.
Systems for treating sewage and industrial wastes typically include preliminary settling tanks that collect a good part of the suspended load of impurities before the clarified effluent is discharged into the receiving waters or is subjected to further treatment in secondary settling tanks.
A settling tank has four functional zones; namely, an inlet zone, a settling zone, a sludge zone and an outlet zone. For high efficiency, inlets must distribute flow and suspend matter as uniformly as possible within the tank. In the sludge zone, sludge is worked into a sump from which it is withdrawn by gravity or by a pumping action. In the outlet zone, the control of outflow is usually effected by a weir attached to one or both sides of an outlet trough or launder which acts as a lateral spillway.
The Anderson U.S. Pat. No. 4,391,704, discloses an outlet zone whose structure and operation is such as to minimize the emission of odoriferous and noxious fumes into the atmosphere. Because incoming waste is saturated with noxious gases and easily volatilized compounds, some degree of odor control is generally necessary in a liquid waste treatment system. In sewage, the objectionable compound which predominates is hydrogen sulfide, a colorless gas having an offensive odor. This gas is highly toxic and a strong irritant to eyes and mucous membranes. Hydrogen sulfide is almost always present in sewage, for it is the product of anaerobic decomposition by bacteria present in the sewage and in the sewage treatment system.
The Anderson invention is based on the recognition that almost all the noxious gases generated in a settling tank are released as the clarified water spills over the weirs mounted on the sides of the trough; hence by capturing these hazardous gases in the trough region and transporting the captured gases to a gas-control system or in otherwise disposing of the gases, one is able to reduce the discharge of gases into the atmosphere to an acceptably safe level.
The Anderson patent provides an outlet zone for a settling tank in which the trough is covered by a hood that also functions as a scum baffle. In a conventional outlet zone, a baffle which protrudes into the tank water is mounted adjacent to the weir of the trough to prevent grease and other floating matter from being discharged into the trough with the effluent. With the Anderson arrangement there is no need for separate baffle plates; for the hood which acts to confine the gases released in the trough has a baffle integral therewith.
Inasmuch as the hood in Anderson which covers the trough is required to collect a relatively small volume of air as compared to a cover placed over the tank, a significant advantage of this arrangement is that it is not only more effective than a massive tank cover, but is also substantially less expensive to construct and operate.
The arrangement disclosed in the Anderson patent represents a significant contribution to the art of wastewater treatment, yet it has certain practical drawbacks. In Anderson, the weir which permits clarified tank water to spill into the trough lies under the gas-confining hood, at least one side of the hood protruding into the water to function as a scum baffle. After prolonged operation, the weir accumulates algae, sludge and other contaminants, and these interfere with the proper operation of the weir. Hence it becomes necessary, on occasion, to remove the entire hood from the trough in order to gain access to the weir so that it can be scrubbed and cleaned, after which the hood must be placed back on the trough. This is a somewhat difficult, costly and time consuming operation.
And if one wishes only to decontaminate the side of the hood which acts as a scum baffle in order to remove scum build-up without, however, removing the hood from the trough, this operation also presents difficulties. The hood in Anderson is molded of fiberglass-reinforced plastic and has a quasi-convex top surface. This rounded surface is not an easy or safe surface for an operator to walk over, either to obtain access to the scum baffle or to inspect the installation.
The prior 1988 (Roley U.S. Pat. No. 4,767,536) is directed to an improvement over the Anderson arrangement in that access to the weir is obtained without the need to remove the hood which serve to confine and capture noxious gases emitted from the trough. Thus, cleaning of the weir presents no practical difficulties.
The prior Roley patent provides an outlet which includes a walkway that overlies the trough and the weir and serves to confine noxious gases emitted into the region above the trough and weir. This walkway makes it possible for an operator to gain access to a scum baffle so that it can be cleaned or for the operator to inspect the installation, the walkway being provided with a safety hand rail. The walkway includes a hatch cover section affording access to the weir when it becomes necessary to scrub and clean the weir.
It is important when the hatch cover is closed that no fumes are then permitted to escape from the outlet zone to the atmosphere, for these fumes, even in low concentrations in the atmosphere having a disagreeable smell and are noxious. With a conventional hinged hatch cover as in the prior Roley patent, though the cover fits neatly over the hatch opening, it is not sealed along the edges and some gas leakage will occur.